In general, an electrical circuit breaker operates to engage and disengage a selected branch electrical circuit from an electrical power supply. The circuit breaker ensures current interruption thereby providing protection to the electrical circuit from unwanted electrical conditions, such as continuous over-current conditions and high current transients due, for example, to electrical short circuits. Such circuit breakers operate by separating a pair of internal electrical contacts contained within a housing (e.g., molded case) of the circuit breaker.
Typically, one electrical contact is stationary, while the other is movable. Conventional circuit breakers may include a moving electrical contact mounted on an end of a moving (e.g., pivotable) contact arm, such that the moving electrical contact moves through a separation path. Contact separation between the moving and stationary electrical contacts may also occur manually, such as by a person throwing a handle of the circuit breaker.
In the case of a tripping event (e.g., a short circuit), an armature may be de-latched so as to release the contact arm and open the electrical contacts of the circuit breaker. Conventionally, tripping may be accomplished by a tripping mechanism wherein the armature is actuated via attraction to a magnet contained in the current path to cause de-latching of a cradle from the armature according to existing designs.
It is desirable for circuit breakers with low handle ratings (e.g., 15 A, 20 A, and 30 A handle rating circuit breakers), that the threshold tripping condition for a short circuit condition be relatively low. In existing designs, however, the magnet of the bimetal element and magnet assembly only operates at about 150 A or more for a 15 A circuit breaker (about 10× or more than the circuit breaker handle rating), about 150 A or more for a 20 A circuit breaker (about 7.5× or more than the circuit breaker handle rating), and about 300 A or more for a 30 A circuit breaker (about 10× or more than the circuit breaker handle rating).
In one common design of the bimetal element and magnet assembly, the magnet is a U-shaped steel piece, which is magnetized when current passes through the U-shape steel piece. This operates as a magnet and attracts the armature of the circuit breaker to de-latch the armature from the cradle and open the electrical contacts when the current through the U-shape steel piece reaches the so-called “instantaneous level.” In designs including an existing bimetal element and magnet assembly, lowering the instantaneous level of the circuit breaker is a significant challenge.
Accordingly, there is a need for circuit breakers and tripping mechanisms thereof that offer relatively-lower instantaneous levels.